Printing system and method of recording images

ABSTRACT

A system of the present invention first refers to a table of a recording ratio with respect to a higher-density ink and determines whether or not deep dots are to be formed, based on input tone data, according to a systematic dither method. When it is determined that deep dots are formed, the system drives a piezoelectric element PE of an ink head corresponding to the higher-density ink to form deep dots and computes a resulting value RV of the deep dots. When it is determined that deep dots are not formed, the system sets the resulting value RV of the deep dots equal to zero, determines whether or not light dots are to be formed by a lower-density ink, based on the input tone data, according to an error diffusion technique, and computes a resulting value RV of the light dots. The on/off control of the light dots minimizes a density error or a difference of a resulting image from an original image. This structure enables a printer for printing images with a plurality of dots having different densities per unit area to appropriately determine the on/off state of the plurality of dots and thereby improve the printing quality. Another applicable structure makes the existence or non-existence of dots by an achromatic color ink reflect upon formation of dots by cyan ink.

This is a continuation of Ser. No. 09/029,865 filed Mar. 17, 1998, nowU.S. Pat. No. 6,099,105.

TECHNICAL FIELD

The present invention relates to a printing system with a head, whichforms at least two different dots having different densities per unitarea on an object, for recording multi-tone images by the dots formed bythe head, as well as to a method of recording such images.

BACKGROUND ART

Color printers, in which a plurality of color inks are discharged from ahead, are widely used as an output device of a computer that records amulti-color, multi-tone image processed by the computer. Several methodsare applicable to print a multi-color, multi-tone image with three colorinks, cyan, magenta, and yellow (CMY). One method is a technique adoptedin the conventional printers. This technique expresses the tone of aprinted image by the density of dots (frequency of appearance of dotsper unit area) while fixing the size of dots formed on a sheet of paperby a spout of ink. Another method adjusts the diameter of dots formed ona sheet of paper, in order to vary the density of dots per unit area.Although the advanced processing of the head for forming ink particleshas been improving the density of dots formable per predetermined lengthor the variable range of the dot diameter, the printers have only thelimited printing density (resolution) to 300 through 720 dpi and thelimited particle diameter to several ten microns. The resolution ofprinters is significantly lower than the resolution of silverphotography, which has reached several thousand dpi on the film.

Dots are sparsely formed in an area of low image density, that is, in anarea of low density of dots to be printed. This increases the degree ofgranularity and makes the dots conspicuous. A printing system and amethod utilizing inks of different densities have been proposed toimprove the printing quality. The proposed technique utilizes ahigh-density ink and a low-density ink for a certain color and regulatesdischarge of these inks, thereby realizing print with an excellent toneexpression. By way of example, a method of and an apparatus forrecording multi-tone images are disclosed in JAPANESE PATENT LAYING-OPENGAZETTE No. 61-108254. The disclosed technique provides a head forforming deep dots and light dots for a specific color and regulates thenumber and overlap of deep dots and light dots formed in a predetermineddot matrix according to the density information of an input image, so asto record a multi-tone image.

The proposed printing system utilizing inks of different densities,however, does not give any specific idea on the allocation ofhigh-density ink and low-density ink to the input tone signals of anoriginal image. Inks of different densities are simply allocated in theorder of densities to the input tone signals of the image (for example,FIG. 9 in JAPANESE PATENT LAYING-OPEN GAZETTE No. 2-215541).

In a printing system for forming at least two different dots havingdifferent densities per unit area (for example, dots by at least twoinks of different densities), the object of the present invention is toenable the at least two different dots to be appropriately mapped totone signals of an original image, thereby improving the quality of aresulting recorded image.

Disclosure of the Invention

The present invention applies the following structures, in order torealize at least part of the above and the other related objects. Thepresent invention is directed to a first printing system with a head,which forms at least two different dots having different densities perunit area on an object, for recording a multi-tone image by adistribution of the dots, the first printing system including:

input means for successively receiving a tone signal of each targetpixel included in an image to be printed;

recording density setting means for specifying a recording density to berealized by at least a selected one of a dot having a higher density perunit area, that is, a higher-density dot, and a dot having a lowerdensity per unit area, that is, a lower-density dot, which are bothincluded in the at least two different dots having different densitiesper unit area, based on the input tone signal;

first dot formation determination means for carrying out a multivaluingoperation based on the specified recording density, and determiningwhether or not the selected one of the higher-density dot and thelower-density dot is to be formed;

second dot formation determination means for making a result of themultivaluing operation by the first dot formation determination meansreflect upon a recording density to be realized by the other one of thehigher-density dot and the lower-density dot, causing the other one ofthe higher-density dot and the lower-density dot to be subjected to amultivaluing operation according to the reflected recording density, anddetermining whether or not the other one of the higher-density dot andthe lower-density dot is to be formed; and

head driving means for driving the head based on results of thedetermination by the first dot formation determination means and thesecond dot formation determination means, in order to form the at leasttwo different dots having different densities per unit area.

In the first printing system of the present invention, the recordingdensity setting means specifies a recording density to be realized by atleast a selected one of a higher-density dot and a lower-density dot,which are both included in the at least two different dots havingdifferent densities per unit area. The first dot formation determinationmeans carries out a multivaluing operation based on the specifiedrecording density, and determines whether or not the selected one of thehigher-density dot and the lower-density dot is to be formed. Themultivaluing operation may be binary coding, ternary coding, orhigher-value coding. In case that the at least two different dots havingdifferent densities per unit area consist of only two different dots,the first dot formation determination means carries out thedetermination for either one of the two different dots. In case that thehead can form four different dots, however, the first dot formationdetermination means may carry out the multivaluing operationsimultaneously for two or more different dots. The second dot formationdetermination means makes the result of the multivaluing operation bythe first dot formation determination means reflect upon a recordingdensity to be realized by the other one of the higher-density dot andthe lower-density dot. The second dot formation determination means thencauses the other one of the higher-density dot and the. lower-densitydot to be subjected to a multivaluing operation according to thereflected recording density, and determines whether or not the other oneof the higher-density dot and the lower-density dot is to be formed. Thehead driving means drives the head based on the results of thedetermination by the first dot formation determination means and thesecond dot formation determination means, in order to form the at leasttwo different dots having different densities per unit area.

This structure enables a density error due to formation of the selectedone of the higher-density dot and the lower-density dot to be reduced byformation of the other one of the higher-density dot and thelower-density dot, so that the tone of an original image is reproducedby a combination of the at least two different dots.

This printing system may be carried out by a variety of embodiments. Byway of example, the recording density setting means may specify only arecording density to be realized by the selected one of thehigher-density dot and the lower-density dot, or alternatively mayspecify a recording density to be realized by the selected one of thehigher-density dot and the lower-density dot as well as a recordingdensity to be realized by the other one of the higher-density dot andthe lower density dot, based on the input tone signal. In the lattercase, the first printing system further includes recording densitycorrecting means for computing correction data, which is to reflect uponthe recording density to be realized by the other one of thehigher-density dot and the lower-density dot, based on the result of themultivaluing operation with respect to the selected one of thehigher-density dot and the lower-density dot, in order to correct therecording density to be realized by the other one of the higher-densitydot and the lower-density dot. The second dot formation determinationmeans determines whether or not the other one of the higher-density dotand the lower-density dot is to be formed, based on the correctedrecording density.

In accordance with one preferable application of the first printingsystem of the present invention, the first dot formation determinationmeans determines whether or not a first dot selected among the at leasttwo different dots having different densities per unit area is to beformed, based on the input tone signal, prior to determination forformation of the other dots. When no formation of the first dot isdetermined, the second dot formation determination means determineswhether or not a second dot having a different density per unit areafrom that of the first dot is to be formed. In this application, thefirst printing system further includes error diffusion means, whichcomputes a difference between a printing density corresponding to theinput tone signal and a printing density realized by the formed dots asa density error, based on the determination of dot formation by thefirst dot formation determination means and the second dot formationdetermination means. The error diffusion means then distributes thedensity error to peripheral pixels in the vicinity of a current targetpixel of dot formation, in order to reflect upon the determination ofdot formation with respect to the peripheral pixels by the first dotformation determination means and the second dot formation determinationmeans.

The present invention is also directed to a second printing system witha head, which forms at least two different dots having differentdensities per unit area on an object, for recording a multi-tone imageby a distribution of the dots, the second printing system including:

input means for successively receiving a tone signal of each targetpixel included in an image to be printed;

tone value setting means for specifying a first dot tone value, that is,a tone value to be realized by a first dot selected among the at leasttwo different dots having different densities per unit area, based onthe input tone signal;

first dot formation determination means for determining whether or notthe first dot is to be formed, based on the first dot tone value;

correction signal computing means for computing a correction signal byadding quantization errors distributed from peripheral processed pixelsin the vicinity of the target pixel to the input tone signal;

second dot formation determination means for, when the first dotformation determination means determines no formation of the first dot,determining whether or not a second dot having a different density perunit area from that of the first dot is to be formed, based on thecorrection signal;

head driving means for driving the head based on results of thedetermination by the first dot formation determination means and thesecond dot formation determination means, in order to form the at leasttwo different dots having different densities per unit area; and

error diffusion means for computing a quantization error, which is adifference between the correction signal and a tone value realized bythe formed dots, as a density error, based on the results of thedetermination by the first dot formation determination means and thesecond dot formation determination means, and distributing and diffusingthe computed density error to peripheral pixels in the vicinity of thetarget pixel.

In the second printing system of the present invention, the input meanssuccessively receives a tone signal of each target pixel included in animage to be printed. The tone value setting means specifies a first dottone value, that is, a tone value to be realized by a first dot selectedamong the at least two different dots having different densities perunit area, based on the input tone signal. The first dot formationdetermination means determines whether or not the first dot is to beformed, based on the first dot tone value. The correction signalcomputing means computes a correction signal by adding quantizationerrors distributed from peripheral processed pixels in the vicinity ofthe target pixel to the input tone signal. The second dot formationdetermination means carries out the processing, based on the correctionsignal. When the first dot formation determination means determines noformation of the first dot, the second dot formation determination meansdetermines whether or not a second dot having a different density perunit area from that of the first dot is to be formed. The head drivingmeans drives the head based on results of the determination by the firstdot formation determination means and the second dot formationdetermination means, in order to form the at least two different dotshaving different densities per unit area. The error diffusion meanscomputes a quantization error, which is a difference between thecorrection signal and a tone value realized by the formed dots, as adensity error, based on the results of the determination by the firstdot formation determination means and the second dot formationdetermination means, and distributes and diffuses the computed densityerror to peripheral pixels in the vicinity of the target pixel.

This structure enables a density error due to formation of the first dotto be reduced by formation of the second dot having a different densityper unit area from that of the first dot, so that the tone of anoriginal image is reproduced by a combination of the at least twodifferent dots.

In accordance with one preferable application of either one of the firstand the second printing systems, the second dot formation determinationmeans includes: local effect computing means for calculating a localeffect from the recording density of the selected dot, which issubjected to determination of dot formation by the first dot formationdetermination means, and a printing density realized by the selecteddot; and recording density correcting means for correcting the recordingdensity to be realized by the other dot by taking into account the localeffect, so as to affect the determination of dot formation with respectto the other dot. In this structure, the local effect with respect tothe selected dot affects the determination of dot formation with respectto the other dot. This structure makes it difficult to form thelower-density dot in a pixel where the higher-density dot has alreadybeen formed, thereby reducing the possibility of polarized formation ofthe at least two different dots having different densities per unitarea. This improves the printing quality.

In accordance with one preferable embodiment, the local effect computingmeans calculates a difference between the recording density of theselected dot and the printing density realized by the selected dot as alocal error, and the recording density correcting means adds a productof the local error and a predetermined weight to the recording densityto be realized by the other dot, so as to affect the determination ofdot formation with respect to the other dot.

The first dot formation determination means may carry out thedetermination for the dot having a higher density per unit area oralternatively for the dot having a lower density per unit area. Thefirst dot subjected to the determination by the first dot formationdetermination means may depend upon the dot formation technique (forexample, the error diffusion method or the systematic dither method) aswell as the properties of an image to be printed. When the first dotformation determination means adopts a dither method, a systematicdither method is preferably carried out with a threshold matrix ofdiscrete dither.

The head may discharge at least two inks of different densities to formthe at least two different dots having different densities per unitarea. It is preferable that the at least two inks of different densitiesinclude a higher-density ink and a lower-density ink and that a dyedensity of the lower-density ink is approximately one quarter a dyedensity of the higher-density ink.

When the head discharges a plurality of chromatic color inks to formchromatic dots as well as an achromatic color ink, such as black ink, toform an achromatic dot, one preferable structure of the printing systemincludes third dot formation determination means for determining whetheror not the achromatic dot is to be formed by the achromatic color ink.When the third dot formation determination means determines formation ofthe achromatic dot by the achromatic color ink, the preferable structureassumes that the first dot formation determination means determinesformation of the selected dot and activates the second dot formationdetermination means and the error diffusion means. This is because theachromatic color is considered to include the components of thechromatic colors in multi-color printing.

In case that the achromatic dot is formed by the achromatic color ink,the degree of effect of the achromatic dot on a density error withrespect to the chromatic color inks may be specified for each chromaticcolor. The density error may be distributed to density errors withrespect to the respective chromatic color inks.

The head of such a printing system discharges two inks of differentdensities for at least either of cyan and magenta, in order to enablecolor printing.

The present invention is further directed to a third printing systemwith a head, which forms at least two different dots having differentdensities per unit area by a chromatic color ink as well as anachromatic dot by an achromatic color ink on an object, for recording amulti-tone image by a distribution of the dots, the third printingsystem including:

input means for successively receiving a tone signal of each targetpixel included in an image to be printed:

density specifying means for specifying a density to be realized by thechromatic color ink and a density to be realized by the achromatic colorink, based on the input tone signal;

achromatic dot formation determination means for carrying out amultivaluing operation for the achromatic color ink, based on thedensity to be realized by the achromatic color ink, and determiningwhether or not the achromatic dot is to be formed by the achromaticcolor ink;

density correcting means for obtaining correction data, which reflectsupon the density to be realized by the chromatic color ink, based on theresult of the multivaluing operation with respect to the achromaticcolor ink, in order to correct the density to be realized by thechromatic color ink;

chromatic dot formation determination means for carrying out amultivaluing operation with respect to the at least two different dotshaving different densities per unit area, based on the corrected densityto be realized by the chromatic color ink, and determining whether ornot the at least two different dots are to be formed; and

head driving means for driving the head based on results of thedetermination by the achromatic dot formation determination means andthe chromatic dot formation determination means, in order to form the atleast two different dots having different densities per unit area by thechromatic color ink and the achromatic dot by the achromatic color ink.

In the third printing system according to the present invention,formation of the achromatic dot by the achromatic color affects theformation of the at least two different dots having different densitiesper unit area by the chromatic color. The achromatic dot is consideredto include the component of the chromatic color. This structure thusappropriately controls the on/off state of the chromatic dot, based onthe on/off state of the achromatic dot (for example, black dot).

The present invention is also directed to a fourth printing system witha head, which forms at least two different dots having differentdensities per unit area on an object, for recording a multi-tone imageby a distribution of the dots, the fourth printing system including:

input means for successively receiving a tone signal of each targetpixel included in an image to be printed;

first dot formation determination means for carrying out a multivaluingoperation with respect to a selected dot among the at least twodifferent dots having different densities per unit area, based on theinput tone signal, and determining whether or not the selected dot is tobe formed;

difference computing means for computing a difference between the inputtone signal and a printing density realized by the selected dot;

second dot formation determination means for carrying out a multivaluingoperation with respect to another dot among the at least two differentdots having different densities, based on the difference, anddetermining whether or not the another dot is to be formed; and

head driving means for driving the head based on results of thedetermination by the first dot formation determination means and thesecond dot formation determination means, in order to form the at leasttwo different dots having different densities per unit area.

The fourth printing system of the present invention first carries out amultivaluing operation with respect to a selected dot among the at leasttwo different dots having different densities per unit area. The systemthen carries out a multivaluing operation with respect to another dotamong the at least two different dots having different densities, basedon a difference between the input tone signal and a printing densityrealized by the selected dot. The head is subsequently driven to formthe at least two different dots having different densities per unitarea.

In accordance with one preferable application of the fourth printingsystem, the difference computing means includes:

first effect computing means for calculating a first effect on themultivaluing operation with respect to the another dot, based on thetone signal and a recording density realized by the selected dot; and

second effect computing means for calculating a second effect on themultivaluing operation with respect to the another dot, based on theprinting density realized by the selected dot,

the difference computing means computing the difference by taking intoaccount the first effect and the second effect.

In this structure, the difference is computed by taking into account thedegree of these first effect and the second effect. Varying the degreeof these effects reflects upon the determination of formation ornon-formation of the another dot based on the difference by the seconddot formation determination means.

In accordance with another preferable application, the fourth printingsystem further includes error diffusion means for computing a differencebetween a printing density realized by the another dot based on theinput tone signal and a printing density realized by the another dot asa density error, based on the determination of dot formation by thesecond dot formation determination means, and distributing the densityerror to peripheral pixels in the vicinity of a current target pixel ofdot formation, in order to reflect upon the determination of dotformation with respect to the peripheral pixels by the second dotformation determination means. This structure ensures the advantages ofthe error diffusion technique (reduction of the mean density error andimprovement in printing quality).

In the fourth printing system of the present invention, the first dotformation determination means or the second dot formation determinationmeans may determine formation or non-formation of the dot by a dithermethod. When the dither method is adopted, a threshold matrix ofdiscrete dither is preferably used.

In accordance with one preferable application, the head forms at leasttwo different dots of different diameters as the at least two differentdots having different densities per unit area. This applicationcorresponds to a printing system with a mechanism of varying the dotdiameter. Other possible structures to form the at least two differentdots having different densities per unit area include a structure usingat least two inks of different dye densities and a structure forchanging the number of times of discharging ink of a fixed density atsubstantially the same place.

The head for forming the at least two different dots having differentdensities or different diameters may have a mechanism for dischargingink particles under a pressure applied to each ink running through anink conduit by application of a voltage to a piezoelectric elementarranged in the ink conduit. Alternatively the head may have a mechanismfor discharging ink particles under a pressure applied to each inkrunning through an ink conduit by air bubbles that are produced by asupply of electricity to a heating body arranged in the ink conduit.These mechanisms give very fine ink particles and enable adequateregulation of the amount of each ink. A number of nozzles for sprayingthe ink particles may be formed on the head. In this case, a pluralityof nozzles are arranged in a feeding direction of a sheet of paper, onwhich the multi-tone image is printed, for each color ink of eachdensity. This structure enhances the printing rate.

The present invention is also directed to a first method of recording amulti-tone image by a distribution of at least two different dots havingdifferent densities per unit area, which are formed on an object by ahead, the first method including the steps of:

successively receiving a tone signal of each target pixel included in animage to be printed;

specifying a recording density to be realized by at least a selected oneof a dot having a higher density per unit area, that is, ahigher-density dot, and a dot having a lower density per unit area, thatis, a lower-density dot, which are both included in the at least twodifferent dots having different densities, based on the input tonesignal;

carrying out a multivaluing operation based on the specified recordingdensity, and determining whether or not the selected one of thehigher-density dot and the lower-density dot is to be formed;

making a result of the multivaluing operation with respect to theselected dot reflect upon a recording density to be realized by theother one of the higher-density dot and the lower-density dot, causingthe other one of the higher-density dot and the lower-density dot to besubjected to a multivaluing operation according to the reflectedrecording density, and determining whether or not the other one of thehigher-density dot and the lower-density dot is to be formed; and

driving the head based on results of the determination with respect tothe selected dot and the other dot, in order to form the at least twodifferent dots having different densities per unit area.

This structure enables a density error due to formation of the selectedone of the higher-density dot and the lower-density dot to be reduced byformation of the other one of the higher-density dot and thelower-density dot, so that the tone of an original image is reproducedby a combination of the at least two different dots.

In accordance with one preferable application, the first methoddetermines whether or not a first dot selected among the at least twodifferent dots having different densities per unit area is to be formed,based on the input tone signal, prior to determination for formation ofthe other dots. When no formation of the first dot is determined, thefirst method determines whether or not a second dot having a differentdensity per unit area from that of the first dot is to be formed. Inthis application, the first method then computes a difference between aprinting density corresponding to the input tone signal and a printingdensity realized by the formed dots as a density error, based on thedetermination of dot formation. The density error is distributed toperipheral pixels in the vicinity of a current target pixel of dotformation, in order to reflect upon the determination of dot formationwith respect to the peripheral pixels.

This structure enables a density error due to formation of the selectedone of the higher-density dot and the lower-density dot to be reduced byformation of the other one of the higher-density dot and thelower-density dot, so that the tone of an original image is reproducedby a combination of the at least two different dots.

The present invention is further directed to a second method ofrecording a multi-tone image by a distribution of at least two differentdots having different densities per unit area, which are formed on anobject by a head, the second method including the steps of:

successively receiving a tone signal of each target pixel included in animage to be printed;

specifying a first dot tone value, that is, a tone value to be realizedby a first dot selected among the at least two different dots havingdifferent densities per unit area, based on the input tone signal;

determining whether or not the first dot is to be formed, based on thefirst dot tone value;

computing a correction signal by adding quantization errors distributedfrom peripheral processed pixels in the vicinity of the target pixel tothe input tone signal;

when no formation of the first dot is determined according to the firstdot tone value, determining whether or not a second dot having adifferent density per unit area from that of the first dot is to beformed, based on the correction signal;

driving the head based on results of the determination with respect tothe first dot and the second dot, in order to form the at least twodifferent dots having different densities per unit area; and

computing a quantization error, which is a difference between thecorrection signal and a tone value realized by the formed dots, as adensity error, based on the results of the determination with respect tothe first dot and the second dot, and distributing and diffusing thecomputed density error to peripheral pixels in the vicinity of thetarget pixel.

The second method of the present invention successively receives a tonesignal of each target pixel included in an image to be printed, andspecifies a first dot tone value, that is, a tone value to be realizedby a first dot selected among the at least two different dots havingdifferent densities per unit area, based on the input tone signal. It isthen determined whether or not the first dot is to be formed, based onthe first dot tone value. The second method then computes a correctionsignal by adding quantization errors distributed from peripheralprocessed pixels in the vicinity of the target pixel to the input tonesignal, and carries out the processing, based on the correction signal.When no formation of the first dot is determined, it is then determinedwhether or not a second dot having a different density per unit areafrom that of the first dot is to be formed. The head is driven, based onresults of the determination of dot formation, so as to form the atleast two different dots having different densities per unit area. Thesecond method computes a quantization error, which is a differencebetween the correction signal and a tone value realized by the formeddots, as a density error, based on the results of the determination ofdot formation, and distributes and diffuses the computed density errorto peripheral pixels in the vicinity of the target pixel.

This structure enables a density error due to formation of the first dotto be reduced by formation of the second dot having a different densityper unit area from that of the first dot, so that the tone of anoriginal image is reproduced by a combination of the at least twodifferent dots.

The present invention also includes some other applications. The firstapplication is a structure, in which one or related ones of the inputmeans, the error diffusion means, the first dot formation determinationmeans, the correction signal computing means, and the second dotformation determination means are not included in the casing of theprinting system but in an apparatus for outputting images to be printed.The error diffusion method as well as the first dot formationdetermination means and the second dot formation determination means maybe realized by discrete circuits or alternatively by the software in anarithmetic and logic circuit including a CPU. In the latter case, theapparatus for outputting images to be printed, such as a computer,carries out the processing related to generation of dots. Only amechanism for regulating discharge of inks from the head to actuallyform the generated dots, for example, on a sheet of paper, is disposedin the casing of the printing system. Another possible structure dividesthese required means into two groups, and enables one group to berealized in the casing of the printing system and the other group to berealized in the apparatus for outputting images.

The second application is a portable recording medium, on which thesoftware loaded to the computer system for execution is recorded. Atleast part of the input means and the dot formation determination meansmay be realized by an arithmetic and logic circuit including a CPU(hardware) and a software program executed thereon. At least part of thesoftware program is stored on the portable recording medium.

The third application is a program supply apparatus for supplying thesoftware program via a communications line.

The fourth application is an ink cartridge used in any one of theprinting systems of the present invention. In color printing with atleast two inks of different dye densities to form at least two differentdots having different densities per unit area, in accordance with onepreferable structure, the ink cartridge includes a black ink cartridgeand a color ink cartridge that is separate from the black ink cartridgeand reserves a plurality of color inks including at least two inks ofdifferent densities. This structure allows the black ink cartridge thatis more frequently used for printing characters and the color inkcartridge to be replaced at arbitrary timings.

In the ink cartridge used for the printing system with the inks ofdifferent densities, at least two inks having an identical hue butdifferent densities are arranged adjacent to each other. In accordancewith a concrete structure, cyan ink, ink having a lower dye density thanthe cyan ink, magenta ink, ink having a lower dye density than themagenta ink, and yellow ink are arranged in this sequence in the inkcartridge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates structure of a printer 20 embodying thepresent invention;

FIG. 2 is a block diagram showing structure of a control circuit 40included in the printer 20;

FIG. 3 is a perspective view illustrating structure of a carriage 30;

FIG. 4 shows an arrangement of color ink heads 61 through 66 in a printhead 28;

FIG. 5 is a perspective view showing a color ink cartridge 70;

FIG. 6 shows a mechanism of ink discharge in each of the color ink heads61 through 66;

FIG. 7 shows a process of discharging ink particles Ip by extension of apiezoelectric element PE;

FIG. 8 is a block diagram showing a series of processes that enable thecomputer 90 to print images based on image information;

FIG. 9 shows compositions of color inks used in the embodiment;

FIG. 10 is a graph showing the lightness plotted against the recordingratio of each color ink;

FIG. 11 is a flowchart showing a processing routine executed by thehalftone module 99;

FIG. 12 shows weights added to the peripheral pixels, into which anerror is distributed in the process of error distribution;

FIG. 13 is a graph showing the relationship between the tone data andthe recording ratios of light ink and deep ink in the embodiment;

FIG. 14 is a flowchart showing a routine of determining formation ornon-formation of deep dots;

FIG. 15 is a flowchart showing a routine of determining formation ornon-formation of light dots;

FIG. 16 shows a technique of determining the on/off state of deep dotsby the systematic dither method;

FIG. 17 is a graph showing the threshold value Dref2 plotted against thecorrected data DC;

FIG. 18 is a graph for determining the resulting value RV when theon/off state of deep dots or that of light dots is specified;

FIG. 19 is a graph for determining the resulting value RV of light dotsbased on the density Sn of deep dots and the density St of light dots;

FIG. 20 shows the process of recording dots by a higher-density ink anda lower-density ink;

FIG. 21 shows a process of recording dots of different diameters;

FIG. 22 is a flowchart showing an image recording process routineexecuted in a third embodiment according to the present invention;

FIG. 23 is a flowchart showing a binary coding process for black ink inthe third embodiment;

FIG. 24 is a flowchart showing a halftone processing routine for cyan(magenta);

FIG. 25 is a flowchart showing a halftone processing routine executed ina fourth embodiment according to the present invention;

FIG. 26 is a flowchart showing a binary coding process for black ink inthe fourth embodiment;

FIG. 27 is a flowchart showing a ternary coding process for the cyancomponent in the fourth embodiment;

FIG. 28 is a flowchart showing an essential part of a fifth embodimentaccording to the present invention;

FIG. 29 is a flowchart showing a ternary coding process for cyan ink Cin the fifth embodiment;

FIG. 30 is a flowchart showing a halftone processing routine executed ina sixth embodiment according to the present invention;

FIG. 31 illustrates an internal structure of the computer 90 andconnection thereof with a network; and

FIG. 32 shows another mechanism for discharging ink particles.

BEST MODES FOR CARRYING OUT THE INVENTION

Some modes of carrying out the present invention are discussed below aspreferred embodiments. FIG. 1 schematically illustrates structure of aprinter 20 as an embodiment according to the present invention. Theprinter 20 has a mechanism for feeding a sheet of paper P by means of asheet feed motor 22, a mechanism for reciprocating a carriage 30 alongthe axis of a platen 26 by means of a carriage motor 24, a mechanism fordriving a print head 28 mounted on the carriage 30 to control dischargeof ink and production of dot patterns, and a control circuit 40 fortransmitting signals to and from the sheet feed motor 22, the carriagemotor 24, the print head 28, and a control panel 32.

The mechanism for feeding the sheet of paper P has a gear train (notshown) for transmitting rotations of the sheet feed motor 22 to theplaten 26 as well as a sheet feed roller (not shown). The mechanism forreciprocating the carriage 30 includes a sliding shaft 34 arranged inparallel with the axis of the platen 26 for slidably supporting thecarriage 30, a pulley 38, an endless drive belt 36 spanned between thecarriage motor 24 and the pulley 38, and a position sensor 39 fordetecting the position of the origin of the carriage 30.

The following describes the structure of the control circuit 40 and theperipheral units included in the printer 20. Referring to the blockdiagram of FIG. 2, the control circuit 40 is constructed as a knownarithmetic and logic operation circuit including a CPU 41, a P-ROM 43for storing programs, a RAM 44, and a character generator (CG) 45 forstoring dot matrices of characters. The control circuit 40 furtherincludes an exclusive I/F circuit 50 exclusively working as an interfaceto an external motor and the like, a head drive circuit 52 connectedwith the exclusive I/F circuit 50 for driving the print head 28, and amotor drive circuit 54 connected with the exclusive I/F circuit 30 fordriving the sheet feed motor 22 and the carriage motor 24. The exclusiveI/F circuit 50 includes a parallel interface circuit and is connected toa computer via a connector 56 to receive printing signals output fromthe computer. Output of image signals from the computer will bediscussed later.

The following describes a concrete structure of the carriage 30 and theprinciple of discharging ink by the print head 28 mounted on thecarriage 30. FIG. 3 is a perspective view showing structure of thecarriage 30. FIG. 4 is a plan view illustrating nozzles arranged in theprint head 28 set on the lower portion of the carriage 30 for sprayingthe respective color inks. A black ink cartridge and a color inkcartridge 70 (see FIG. 5) are attachable to the substantially L-shapedcarriage 30 shown in FIG. 3. A partition wall 31 separates the black inkcartridge from the color ink cartridge. Six color ink heads 61 through66 for respectively discharging color inks are formed in the print head28 that is disposed on the lower portion of the carriage 30. Ink supplypipes 71 through 76 for leading inks from ink tanks to the respectivecolor ink heads 61 through 66 are formed upright on the bottom of thecarriage 30 as shown in FIG. 3. When the black ink cartridge and thecolor ink cartridge 70 are attached downward to the carriage 30, the inksupply pipes 71 through 76 are inserted into connection apertures (notshown) formed in the respective cartridges.

When the ink cartridge 70 is attached to the carriage 30, inks in theink cartridge 70 are sucked out by capillarity through the ink supplypipes 71 through 76 and are led to the color ink heads 61 through 66formed in the print head 28 arranged on the lower portion of thecarriage 30 as shown in FIG. 6. In case that the ink cartridge 70 isattached to the carriage 3 0 f or the first time, a pump works to suckinks into the respective color ink heads 61 through 66. In thisembodiment, structures of the pump for suction and a cap for coveringthe print head 28 during the suction are not illustrated nor describedspecifically.

A row of thirty-two nozzles ‘n’ are formed in each of the color inkheads 61 through 66 as shown in FIGS. 4 and 6. A piezoelectric elementPE having excellent response, which is one of electrically distortingelements, is arranged for each row of nozzles ‘n’. FIG. 7 illustrates aconfiguration of the piezoelectric element PE and the nozzles ‘n’. Thepiezoelectric element PE is disposed at a position that comes intocontact with an ink conduit 80 for leading ink to the nozzles ‘n’. As isknown, the piezoelectric element PE has a crystal structure that issubjected to a mechanical stress due to application of a voltage andthereby carries out extremely high-speed conversion of electrical energyto mechanical energy. In this embodiment, application of a voltagebetween electrodes on either ends of the piezoelectric element PE for apredetermined time period causes the piezoelectric element PE toabruptly extend and deform one side wall of the ink conduit 80 as shownin the lower drawing of FIG. 7. The volume of the ink conduit 80 isreduced with an extension of the piezoelectric element PE, and a certainamount of ink corresponding to the volume reduction is sprayed as inkparticles Ip from the ends of the nozzles ‘n’ at a high speed. The inkparticles Ip soak into the sheet of paper P set on the platen 26, so asto print images.

In order to ensure spaces for the piezoelectric elements PE, the sixcolor ink heads 61 through 66 are divided into three pairs on the printhead 28 as shown in FIG. 4. The first pair includes the black ink head61 that is arranged at one end close to the black ink cartridge and thecyan ink head 62 that is disposed next to the black ink head 61. Thesecond pair includes the light cyan ink head 63 for cyan ink having thelower density than that of the standard cyan ink supplied to the cyanink head 62 (hereinafter referred to as light cyan ink.) and the magentaink head 64. The third pair includes the light magenta ink head 65 formagenta ink having the lower density than that of the standard magentaink supplied to the magenta ink head 64 (hereinafter referred to aslight magenta ink) and the yellow ink head 66. The compositions anddensities of the respective inks will be discussed later.

In the printer 20 of the embodiment having the hardware structurediscussed above, while the sheet feed motor 22 rotates the platen 26 andthe other related rollers to feed the sheet of paper P, the carriagemotor 24 drives and reciprocates the carriage 30, simultaneously withactuation of the piezoelectric elements PE on the respective color inkheads 61 through 6.6 of the print head 28. The printer 20 accordinglysprays the respective color inks and transfers multi-color images ontothe sheet of paper P. Referring to FIG. 8, the printer 20 printsmulti-color images based on signals output from an image productionapparatus, such as a computer 90, via the connector 56. In thisembodiment, an applications program 95 working in the computer 90processes images and displays the processed images on a CRT display 93via a video driver 91. When the applications program 95 outputs aprinting instruction, a printer driver 96 in the computer 90 receivesimage information from the applications program 95 and the printer 20converts the image information to printable signals. In the example ofFIG. 8, the printer driver 96 includes a rasterizer 97 for convertingthe image information processed by the applications program 95 todot-based color information, a color correction module 98 for causingthe image information that has been converted to the dot-based colorinformation (tone data) to be subjected to color correction according tothe colorimetric characteristics of an image output apparatus (theprinter 20 in this embodiment) and a halftone module 99 for generatinghalftone image information, which expresses density of a specified areaby the existence or non-existence of ink in each dot unit, from thecolor-corrected image information. Operations of these modules are knownto the skilled in the art and are thus not specifically described herein principle, though the contents of the halftone module 99 will bediscussed later.

As discussed above, the printer 20 of the embodiment has the additionalheads 63 and 65 for light cyan ink and light magenta ink other than thefour heads 61, 62, 64, and 66 for the standard four color inks K, C, M,and Y in the print head 28. As shown in FIG. 9, light cyan ink and lightmagenta ink have lower dye densities than those of standard cyan ink andmagenta ink. Cyan ink of standard density (defined as C1 in FIG. 9)includes 3.6% by weight of Direct blue 99 as a dye, 30% by weight ofdiethylene glycol, 1% by weight of Surfinol 465, and 65.4% by weight ofwater. Light cyan ink (defined as C2 in FIG. 9), on the other hand,includes only 0.9% by weight of Direct blue 99, that is, one quarter thedye density of the cyan ink C1, and 35% by weight of diethylene glycoland 63.1% by weight of water for adjustment of the viscosity. Magentaink of standard density (defined as M1 in FIG. 9) includes 2.8% byweight of Acid red 289 as a dye, 20% by weight of diethylene glycol, 1%by weight of Surfinol 465, and 79% by weight of water. Light magenta ink(defined as M2 in FIG. 9), on the other hand, includes only 0.7% byweight of Acid red 289, that is, one quarter the dye density of themagenta ink M1, and 25% by weight of diethylene glycol and 74% by weightof water for adjustment of the viscosity.

As shown in FIG. 9, yellow ink Y includes 1.8% by weight of DirectYellow 86 as a dye, whereas black ink K includes 4.8% by weight of Foodblack 2 as a dye. All these inks are adjusted to have the viscosity ofapproximately 3 [mPa·s]. Adjustment of the viscosity to thesubstantially identical level enables identical control of thepiezoelectric elements PE for the respective color heads 61 through 66.

FIG. 10 is a graph showing the lightness of these color inks. Theabscissa of FIG. 10 denotes the recording ratio to the recordingresolution of the printer, that is, the proportion of printing dotsformed by the ink particles Ip discharged from the nozzles ‘n’ to thewhite sheet of paper P. The recording ratio=100 represents the state, inwhich the whole surface of the sheet of paper P is covered with the inkparticles Ip. In this embodiment, the light cyan ink C2 hasapproximately one quarter the dye density (percent by weight) of thecyan ink C1. The lightness of the light cyan ink C2 at the recordingratio of 100% is equal to the lightness of the cyan ink C1 at therecording ratio of approximately 35%. This relationship is alsoapplicable to the lightness of the magenta ink M1 and the light magentaink M2. The proportion of recording ratios of different-density inksgiving the identical lightness is defined by the beauty of color mixturein case that the two different-density inks are mixed in print. Inpractice, it is desirable to adjust the proportion in the range of 20%to 50%. This relationship is substantially equivalent to the adjustmentof the dye density (percent by weight) of the lower-density ink (lightcyan ink C2 or the light magenta ink M2) to almost one fifth to onethird the dye density (percent by weight) of the higher-density ink(cyan ink C1 or the magenta ink M1).

The printer 20 of the embodiment carries out the processing in thehalftone module 99 of the printer driver 96 and thereby prints imageswith high-density ink and low-density ink. FIG. 11 is a flowchartshowing outline of the processing executed in the halftone module 99.When a printing process starts, pixels are successively scanned from theupper left corner of one image set as the origin. The halftone module 99receives color-corrected tone data DS (8 bits respectively for C, M, Y,and K) of a target pixel in the order along the scanning direction ofthe carriage 30 from the color correction module 98 (step S100).

The following description is on the assumption that images are printedonly in cyan ink. In the actual state, however, images are printed inmultiple colors; deep dots and light dots of magenta are formed by thehigher-density magenta ink M1 and the lower-density light magenta inkM2, whereas dots of yellow and dots of black are respectively formed bythe yellow ink Y and the black ink K. In case that dots are formed bydifferent color inks in a predetermined area, required control iscarried out to realize the favorable color reproduction by colormixture. By way of example, one available control procedure does notallow dots of different colors to be printed at an identical position.

The program then determines the on/off state of deep dots, based on theinput tone data DS (step S120). The process of determining the on/offstate of deep dots follows a routine of determining formation of deepdots shown in the flowchart of FIG. 14. When the program enters theroutine of FIG. 14, the halftone module 99 refers to a table shown inFIG. 13 and generates deep level data Dth based on the input tone dataDS (step S122). FIG. 13 is a table showing the recording ratios of lightink and deep ink plotted against the tone data of the original image.The tone data DS take the values of 0 to 255 for each color (8 bit-datafor each color), and the magnitude of the tone data is accordinglyexpressed, for example, as 16/256 in the following description. Thetable of FIG. 13 shows the ratio of deep ink to light ink in a resultingprint, and does not unequivocally specify the recording ratios of deepink and light ink against a certain piece of tone data DS to determinethe on/off state of dots by deep ink or light ink in a target pixel. Thefundamental technique shown in the flowchart of FIG. 11 first determinesthe on/off state of deep according to the table of FIG. 13 (step S120)and then determines the on/off state of light dots based on the resultsof determination for deep dots (step S140). Coincidence between theobtained recording ratio of light dots and data in the table of FIG. 13is ascribed to the following reason.

The density of an image per unit of area is expressed by the number ofdeep dots and light dots formed therein. According to the table of FIG.13, the number of deep dots formed per unit of area is expressed as theratio to the maximum density, which corresponds to the value ‘255’, andis hereinafter referred to as Ks. In a similar manner, Us represents thenumber of light dots. In order to enable the density of a resultingimage to be identical with tone data DS of an input image, the followingequation should be satisfied:

DS=Ks×(Evaluation value of deep dots)/255+Us×(Evaluation value of lightdots)/255

Since the evaluation value of deep dots (density of created dots) can beregarded as 255, the table of light dots shown in FIG. 13 depends uponthe table of deep dots and the evaluation value of light dots. In theexample of FIG. 13, data regarding a point that gives the maximumrecording ratio of light dots (tone data=95, deep dot data=18, and lightdot data=122) is substituted into the above equation. This specifies theevaluation value of light dots Z as:

95=18×255/255+122×Z/255

The evaluation value of light dots z given by this equation is equal to160. The evaluation value of deep dots and the evaluation value of lightdots are identical with resulting values RV that are used in theflowchart of determining the on/off state of deep dots and light dotsdiscussed later.

Deep level data Dth (right ordinate in FIG. 13) corresponding to apredetermined recording ratio of deep ink is read from the table of FIG.13, based on the input tone data DS. For example, in case that the inputtone data of cyan represents a solid area of 50/256, the recording ratioof the deep cyan ink C1 is equal to 0%, so that the value of deep leveldata Dth is equal to zero. In case that the input tone data represents asolid area of 95/256, the recording ratio of the deep cyan ink C1 isequal to 7%, so that the value of deep level data Dth is equal to 18. Incase that the input tone data represents a solid area of 191/256, therecording ratio of the deep cyan ink C1 is equal to 75%, so that thevalue of deep level data Dth is equal to 191. In the routine ofdetermining the on/off state of light dots formed by a lighter inkdiscussed below, the corresponding recording ratios of the light cyanink C2 are 36%, 58%, and 0%, respectively.

The deep level data Dth thus obtained is then compared with a thresholdvalue Dref1 (step S124). The threshold value Dref1 is a reference valuefor determining whether or not dots of deep ink should be formed in atarget pixel, and may be fixed to approximately half the maximum of deeplevel data Dth. In this embodiment, systematic dither method using athreshold matrix of discrete dither is applied to set the thresholdvalue. The threshold matrix of discrete dither used here is, forexample, a wide-range matrix of 64×64 in size (blue noise matrix).Different threshold values Dref1 used for determining the on/off stateof deep dots are accordingly set for the respective target pixels. FIG.16 shows the principle of the systematic dither method. Although thematrix shown in FIG. 16 has the size of 4×4 as a matter of convenienceof illustration, the matrix actually used in this embodiment has thesize of 64×64. Threshold values (0 to 255) are specified to have no biasof appearance in any 16×16 areas included in the 64×64 matrix. Thewide-range matrix effectively prevents occurrence of pseudo-contours.The discrete dither ensures high spatial frequencies of dots determinedby the threshold matrix and makes dots sufficiently scattered in anyspecified area. A concrete example of the discrete dither is a Beyer'sthreshold matrix. Application of the discrete dither causes deep dots tobe sufficiently scattered and realizes a non-biased distribution of deepdots and light dots, thereby improving the picture quality. Anothertechnique, for example, density pattern method or pixel distributionmethod, may be applied to determine the on/off state of deep dots.

In case that the deep level data Dth is greater than the threshold valueDref1, the program determines the on state of deep dots in the targetpixel and calculates a resulting value RV (step S126). The resultingvalue RV corresponds to the density of the target pixel (evaluationvalue of deep dots). In the on state of deep dots, that is, when it isdetermined that dots of high-density ink are to be formed in the targetpixel, the value corresponding to the density of the pixel (for example,the value 255) is set as the resulting value RV. The resulting value RVmay be a fixed value or set as a function of deep level data Dth.

In case that the deep level data Dth is not greater than the thresholdvalue Dref1, on the contrary, the program determines the off state ofdeep dots, that is, no formation of dots by high-density ink in thetarget pixel, and sets the value ‘0’ to the resulting value RV (stepS128). Since the white background of the sheet of paper P remains in theplace where no dots of high-density ink are formed, the resulting valueRV is set equal to zero.

After determining the on/off state of deep dots and calculating theresulting value RV (step S120 in FIG. 11), the program calculates lightdot data Dx used for determining the on/off state of light dots (stepS130). Corrected data DC is obtained by adding diffusion errors ΔDudiffused from the peripheral processed pixels to the light dot data Dx(step S135). The light dot data Dx is calculated according to thefollowing equation:

Dx=Dth·Z/255+Dtn·z/255

wherein Dtn denotes light level data read from the table of FIG. 13,based on the tone data DS, Z represents an evaluation value in case ofthe on state of deep dots, and z denotes an evaluation value in case ofthe on state of light dots. The light dot data Dx is the sum of the deeplevel data and the light level data respectively multiplied by weightingcoefficients, which depend upon the respective evaluation values. One ofthe main characteristics of the present invention is that the on/offstate of light dots is determined by the light dot data Dx, which isbased on both the deep level data and the light level data. Since theevaluation value Z of deep dots is equal to 255, the above equation isrewritten as:

Dx=Dth+Dtn·z/255

which is actually used to calculate the light dot data Dx (step S130).The evaluation value z of light dots is significantly smaller than theevaluation value Z of deep dots and is set equal to 160 in thisembodiment.

The corrected data DC is obtained at step S135 by adding the diffusionerrors ΔDu to the light dot data Dx, since the error diffusion iscarried out with respect to the light dots. In the printing process bythe error diffusion method, a density error occurring in a processedpixel is distributed in advance into peripheral pixels near to theprocessed pixel with predetermined weights. The processing of step S135accordingly reads the corresponding errors and causes the errors toaffect the target pixel to be printed next. FIG. 15 illustrates aprocess of distributing the error occurring in a processed pixel PP, forwhich the on/off state of light dots has been determined, intoperipheral pixels with specified weights. The density error isdistributed into several pixels after the processed pixel PP in thescanning direction of the carriage 30 and in the feeding direction ofthe sheet of paper P with predetermined weights (¼, ⅛, {fraction(1/16)}).

After the calculation of the corrected data DC, it is determined whetheror not the pixel is in the off state of deep dots (that is, no dots ofthe cyan ink C1 are formed in the pixel) (step S138). In case of the offstate of deep dots, that is, in case of no formation of deep dots, theprogram determines the on/off state of low-density dots, that is, dotsby the light cyan ink C2 (hereinafter referred to as light dot) (stepS140). The process of determining the on/off state of light dots followsa routine of determining formation of light dots shown in the flowchartof FIG. 16. In this example, the error diffusion method is adopted todetermine the on/off state of light dots, that is, formation of dots bythe light cyan ink C2. The tone data DC corrected according to theconcept of error diffusion is compared with a threshold value Dref2 forlight dots (step S144). The threshold value Dref2 is a reference valuefor determining whether or not dots should be formed by the low-density,light ink in a target pixel. The threshold value Dref2 may be a fixedvalue, but is set as a variable varying with the corrected data DC inthis example. FIG. 17 shows the relationship between the threshold valueDref2 and the corrected data DC. The threshold value Dref2 is set as afunction of the corrected data DC as shown in FIG. 17. This effectivelyprevents delay of dot formation in the vicinity of the lower limit orupper limit of the tone or turbulence of dot formation (a trail of dots)observed in a certain range in the scanning direction under thecondition of an abrupt change in tone in a specified area.

In case that the corrected data DC is greater than the threshold valueDref2,the program determines the on state of light dots and calculates aresulting value RV (evaluation value of light dots) (step S146). Theresulting value RV here has a reference value 122 and is corrected bythe corrected data DC, although it may be a fixed value. In case thatthe corrected data DC is not greater than the threshold value Dref2,onthe contrary, the program determines the off state of light dots andsets the value ‘0’ to the resulting value RV (step S148).

A variety of techniques are applicable to determine the resulting valueRV. One available technique determines the resulting value RV of deepdots based on the deep level data Dth, while determining that of lightdots based on the input data DS. A solid line Jn in FIG. 18 shows afunction applicable to determine the resulting value RV of deep dots,and a broken line Bt in FIG. 18 shows a function applicable to determinethe resulting value RV of light dots. Another possible proceduredetermines the resulting value RV of the second dots which are subjectedto the later determination of the on/off state (the light dots in thisembodiment), based on data regarding both the deep dots and the lightdots. By way of example, the resulting value RV of the light dots may bedetermined as a function of Sn×α+St (α is a coefficient greater than 1),where St and Sn respectively denote a density of light dots and adensity of deep dots, as shown in FIG. 19.

After determining the on/off state of light dots and calculating theresulting value RV (step S140 in FIG. 11), the program calculates anerror (step S150). The error is obtained by subtracting the resultingvalue RV from the corrected data DC. In case that neither deep dots norlight dots are formed, the resulting value RV is equal to zero and thecorrected data DC is set to the error ERR. In this case, no density tobe realized is obtained in the target pixel, so that the density isspecified as the error. In case that either deep dots or light dots areformed, on the other hand, a value corresponding to the created dots isset to the resulting value RV, so that the difference between thecorrected data DC and the resulting value RV is specified as the errorERR.

The program subsequently carries out error diffusion (step S160). Theerror obtained at step S150 is distributed into peripheral pixels aroundthe target pixel with predetermined weights (see FIG. 12). After theerror diffusion process, the program goes to a next pixel and repeatsthe processing of steps S100 through S160 for the next pixel.

FIG. 20 shows the process of recording light dots and deep dots withrespect to the cyan ink C1 and the light cyan ink C2. In the range oflow tone data (in the range of tone data=0/256 to 63/256 in thisembodiment), dots of only the light cyan ink C2 are formed as shown inFIGS. 20a and 20 b. The proportion of light dots existing in apredetermined area increases with an increase in tone data.

In the range of tone data exceeding a predetermined value (in the rangeexceeding 64/256 in this embodiment), formation of deep dots starts andgradually increases while the proportion of light dots still increasesas shown in FIG. 20c. In the range of higher tone data (in the rangeexceeding 95/256 in this embodiment), the proportion of deep dotsincreases while the proportion of Light dots decreases as shown in FIGS.20d and 20 e.

In the range of still higher tone data (in the range exceeding 191/256in this embodiment), no light dots but only deep dots are formed asshown in FIGS. 20f and 20 g. When the tone data reaches the maximum, therecording ratio of deep dots is equal to 100% as shown in FIG. 20h. Inthis state, the whole surface of the sheet of paper P is covered withthe dots of high-density ink (cyan ink C1).

The procedure of the embodiment discussed above first determines whetheror not dots are to be formed by the higher-density ink and specifies theresulting value RV according to the on/off state of the deep dots. Onlyin case of no formation of deep dots, the procedure determines whetheror not dots are to be formed by the lower-density. ink and specifies theresulting value RV according to the on/off state of the light dots. Thesystematic dither method is applied to the determination for the deepdots, whereas the error diffusion method is applied to the determinationfor the light dots. The density of a resulting printed image is thusregulated to minimize the error through the on/off state of the lightdots. Since the determination for the deep dots is carried out prior tothe determination for the light dots, a natural distribution of deepdots having the excellent tone expression can be realized by adequatelysetting the relationship between the input data and the deep level dataDth in the table of FIG. 13.

A simple prior art procedure applied to determine the or/off state ofdeep dots and light dots turns the light dots on when the density ofinput data is within a predetermined range, and turns the deep dots onwhen the density of input data is higher than the predetermined range.This procedure may cause a pseudo-contour when the input data is closeto the upper limit or the lower limit of this predetermined range. Thetechnique of this embodiment, on the other hand, does not cause anypseudo-contours. Another advantage of the embodiment that successivelydetermines the on/off state of deep dots and light dots is to readilyregulate the mixing ratio of the deep dots to the light dots.

In this embodiment, specification of the recording ratios of the lightcyan ink C2 and the cyan ink C1 based on the map of FIG. 13 gives thefollowing characteristics:

(1) Only the light cyan ink C2 is recorded in the range of low tone data(the range of 0/256 to 63/256 in this embodiment) In this range, therecording ratio of the light cyan ink C2 monotonously increases with anincrease in magnitude of tone data.

(2) Before the recording ratio of the light cyan ink C2, which increaseswith an increase in input tone data, reaches its maximum (58% in thisembodiment), formation of dots by the higher-density cyan ink C1 startsand the recording ratio of the cyan ink C1 gradually increases with anincrease in tone data. In this embodiment, formation of dots by the cyanink C1 starts when the value of input tone data exceeds 63/256. Thespecific value of tone data giving the maximum recording ratio of dotsby the light cyan ink C2 is 95/256 in this embodiment.

(3) When the tone data exceeds the specific value that gives the maximumrecording ratio of dots by the light cyan ink C2, the recording ratio ofthe light cyan ink C2 starts decreasing. The recording ratio of the cyanink C1, on the other hand, increases substantially in proportion to anincrease in tone data. In this embodiment, the recording ratio of thelight cyan ink C2 decreases abruptly in the range of tone data exceeding127/256, and is substantially equal to zero in the range of tone dataexceeding 191/256.

(4) In the range of tone data greater than the specific value at whichthe recording ratio of the light cyan ink C2 becomes substantially equalto zero, the recording ratio of the cyan ink C1 gradually increases tothe maximum 100% with an increase in tone data. Compared with theprevious range, however, the increase in recording ratio against theincrease in tone data shows a slightly gentle slope in this range.

In the printer 20 of the embodiment discussed above, deep dots by thehigher-density ink (the cyan ink C1 in the example of FIG. 13) startformation in the range of tone data smaller than the specific value thatgives the maximum recording ratio of light dots by the lower-density ink(the light cyan ink C2 in the example of FIG. 13). This structureenables extremely smooth color mixture on the border between the printwith light dots and the print with deep dots, thereby ensuring theextremely high quality of printing.

This structure also restricts the maximum recording ratio of dots by thelight ink to approximately 60%. No solid state of the light ink in alower tone range effectively prevents occurrence of pseudo-contours inthis tone range. This structure further gives a high degree of freedomto the distribution of dots by the deep ink and ensures naturalexpression in the tone range around the border where the higher-densityink starts mixing with the lower-density ink.

In the range of tone data greater than the specific value that gives themaximum recording ratio of dots by the light ink, the recording ratio ofdots by the light ink abruptly decreases. As the tone data increases,the dots of the light ink are replaced by the dots of the deep ink. Thereplacement decreases the number of ink dots required for expressing acertain tone. This saves the amount of ink discharged for expressing thetone and thereby reduces the total amount of ink used for printing. Therecording ratio of dots by the light ink abruptly decreases and becomessubstantially equal to zero, well before the recording rat-o of dots bythe deep ink reaches 100% (input data=255). This prevents the light inkfrom being used wastefully in the process of printing the high-toneimage area and decreases the total amount of ink discharged forprinting. This structure favorably restricts the amount of ink per unitarea in the sheet of paper.

In the first embodiment, two different dots having different densitiesper unit area are formed by the two inks of the same color but differentdensities. In accordance with one possible application, three or moreinks of the same color but different densities may be applied to thestructure of the above embodiment. In this case, the ratio of dyedensities of these inks may be specified like a geometric series(1:n:2×n: . . . or as a relationship of like powers (1:n²:n⁴. . . . ),wherein n=2,3, . . . (positive integer of not smaller than 2). The firstembodiment applies the systematic dither method to determine the on/offstate of deep dots and the error diffusion method to determine theon/off state of light dots. A variety of other known binarizationtechniques are also applicable to determine the on/off state of deepdots and light dots. Another possible structure gives the priority ofdetermination of the on/off state to the light dots over the deep dots.

Although inks of different densities are used only for cyan and magentain the embodiment, inks of different densities may also be used foryellow and black. Inks of different densities are not restricted to thecombination of C, M, Y, and K but may be applied to another combination.Inks of different densities may be used for special colors, such as goldand silver.

In the first embodiment, dots by the higher-density ink (deep dots) anddots by the lower-density ink (light dots) are formed on the sheet ofpaper P. Similar effects can, however, be attained by forming two ormore different dots in diameter by the same ink having a fixed density.This structure is discussed below as a second embodiment according tothe present invention. The size of the dots formed on the sheet of paperP is controlled by regulating the diameter of the nozzle for dischargingink and the intensity of the voltage pulse (that is, the voltage andduration) applied to the piezoelectric element PE. In the secondembodiment, for example, the nozzle 62 for the cyan ink C1 and thenozzle 63 for the light cyan ink C2 in the first embodiment are replacedrespectively with a nozzle for large-diametral dots and a nozzle forsmall-diametral dots. The control procedure of the first embodiment isadopted in the second embodiment with slight changes, that is,replacement of deep dots by large-diametral dots and light dots bysmall-diametral dots. The structure of the second embodiment determinesthe on/off state of the large-diametral dots according to the input tonedata by the dither method and then determines the on/off state of thesmall-diametral dots based on the error diffusion technique. FIG. 21shows the process of forming the large-diametral dots and thesmall-diametral dots.

The second embodiment exerts similar effects as those of the firstembodiment, that is, smooth tone expression and easy regulation of themixing ratio of the large-diametral dots to the small-diametral dots.Another advantage of the second embodiment is that only one ink isrequired for each color. Formation of the small-diametral dots decreasesthe amount of ink discharged on the sheet of paper P. This isadvantageous from the viewpoint of the ink duty, which represents theallowable amount of ink sprayed per unit area in the sheet of paper P.

The large-diametral dot and the small-diametral dot are not printed atthe same position. One preferable structure accordingly uses only onenozzle for printing both the large-diametral dots and thesmall-diametral dots by varying the intensity of the voltage pulseapplied to the piezoelectric clement PE. This structure decreases thenumber of the nozzles formed in the print head 28 and effectivelyprevents deviation of the printing positions of the large-diametral dotsfrom those of the small-diametral dots. The structure of varying the dotdiameter is also applicable to three or more different dots in diameter.Like the first embodiment, the second embodiment may apply thesystematic dither method to determination for the large-diametral dotsand the error diffusion method to determination for the small-diametraldots. The determination of the on/off state of the respective dots is,however, not restricted to these techniques, and a variety of otherknown binarization techniques are also applicable to determine theon/off state of the large-diametral dots and the small-diametral dots.In this embodiment, the priority of determination of the on/off statemay be given to the large-diametral dots or alternatively to thesmall-diametral dots.

The following describes a third embodiment according to the presentinvention. A printing system of the third embodiment has the samehardware structure as those of the first and the second embodiments, andcan record images with the total of six colors, that is, black ink K,cyan ink C1, light cyan ink C2, magenta ink M1, light magenta ink M2,and yellow ink Y. In the third embodiment, when entering the imagerecording process routine of FIG. 22, the program first receives tonedata of a target pixel (step S200) and executes binary coding for theblack ink (step S210). Del:ails of the binarization for black inkexecuted at step S210 is shown in the flowchart of FIG. 22 and will bediscussed later.

After the binary coding for the black ink, the program successivelycarries out binary coding for the two cyan inks C1 and C2 havingdifferent densities (step S220), binary coding for the two magenta inksM1 and M2 having different densities (step S230), and binary coding forthe yellow ink Y (step S240). Namely binary coding is executed for thetotal of six inks K, C1, C2, M1, M2, and Y, with respect to the targetpixel.

The systematic dither method, which has been discussed in the firstembodiment, is adopted to the binary coding for the black ink as shownin the flowchart of FIG. 23. A wide-range matrix of 64×64 in size (bluenoise matrix) is used to realize binarization with favorabledispersibility for the black ink. After binarization for the black ink,that is, determination of the on/off state of dots formed by the blackink (step S212), it is determined whether or not black dots are ON (stepS214). In the ON state, that is, in case of formation of dots by theblack ink, the value ‘1’ is set to both flags FC and FM (step S216). Inthe OFF state, that is, in case of no formation of dots by the blackink, on the other hand, the value ‘0’ is set to both the flags FC and FM(step S218). These flags FC and FM representing the on/off state of dotsby black ink are referred to in the binary coding process for cyan inkand magenta ink (steps S220 and 230).

The flowchart of FIG. 24 shows details of the halftone processing forcyan ink and magenta ink (steps S220 and S230). The halftone processingfor cyan or magenta includes similar steps to those of the halftoneprocessing discussed in the first embodiment (see FIG. 11). Steps ofFIG. 24 identical with or similar to those of FIG. 11 have like numeralsin the lower two places. The flowchart of FIG. 24 mainly relates to theprocessing for cyan ink, and that for magenta ink is shown inparentheses. When the program enters the routine of FIG. 24, it is firstdetermined whether or not the flag FC is equal to one (step S313). Inthe processing routine for magenta ink, it is here determined whether ornot the flag FM is equal to one. In case that the flag FC (or FM) is notequal to one, the program recognizes the off state of dots by the blackink in the binary coding process for black ink (FIG. 23). Like the firstembodiment, the program then determines the on/off state of deep dots(the cyan ink C1 or the magenta ink M1) and calculates a resulting valueRVC (RVM) (step S320). Corrected data DCC for cyan (DCM for magenta) isobtained by adding diffusion errors ΔDu diffused from the processedpixels, which are in proximity to a target pixel (step S325).

In case that the dots by the black ink are ON in the binary codingprocess for black ink (FC=FM=1), on the other hand, the programconsiders deep dots formed by the cyan ink C1 (magenta ink M1) to bealso in the on state irrespective of the input tone data, and calculatesthe resulting value RVC (RVM) (step S315). When dots are formed by theblack ink K, it can be thought that cyan and magenta exist in black inkaccording to the concept of subtractive mixture of color stimuli. Thereis accordingly no necessity of newly forming dots of cyan or magenta inkupon dots of black ink. The program thus considers dots by the cyan inkC1 and magenta ink M1 to be also in the on state and sets apredetermined value (rvck or rvmk) to the resulting value RVC (RVM).Like step S325, the program obtains corrected data DCC for cyan (DCM formagenta) by adding diffusion errors ΔDu diffused from the processedpixels, which are in proximity to a target pixel (step S318).

In case of no formation of dots by the black ink K (when FC=FM=0), afterobtaining the corrected data DCC (DCM), the program determines whetheror not deep dots are in the on state (that is, whether or not dots areformed by the cyan ink C1 or the magenta ink M1) (step S330). In casethat no deep dots are formed, the program determines the on/off state oflow-density dots (hereinafter referred to as light dots), that is, dotsformed by the light cyan ink C2 (or the light magenta ink M2) (stepS340). The process of determining the on/off state of light dots issimilar to that of the first embodiment (FIG. 15) and thus notspecifically illustrated here. In this embodiment, the error diffusionmethod is adopted to determine formation of light dots by the light cyanink C2 (or the light magenta ink M2). In accordance with a concreteprocedure, the tone data DCC (DCM) corrected according to the concept oferror diffusion is compared with a threshold value Dref2 for light dots.The threshold value Dref2 is a reference value for determining whetheror not dots should be formed by the lower-density, light ink in a targetpixel.

In case that the corrected data DCC (DCM) is greater than the thresholdvalue Dref2,the program determines the on state of light dots andcalculates a resulting value RVC (RVM), which corresponds to anevaluation value of light dots. In case that the corrected data DCC(DCM) is not greater than the threshold value Dref2,on the other hand,the program determines the off state of light dots and sets the value‘0’ to the resulting value RVC (RVM).

After determining the on/off state of light dots and calculating theresulting value RVC (RVM) (step S340), the program calculates an error(step S350). The error is obtained by subtracting the resulting valueRVC (RVM) from the corrected data DCC(DCM). In case that neither deepdots nor light dots are formed, the resulting value RVC (RVM) is equalto zero and the corrected data DCC (DCM) is set to the error ERR. Inthis case, no density to be realized is obtained in the target pixel, sothat the density is specified as the error ERR. In case that either deepdots or light dots are formed, on the other hand, a value correspondingto the created dots is set to the resulting value RVC (RVM), so that thedifference between the corrected data DCC (DCM) and the resulting valueRVC (RVM) is specified as the error ERR. When dots are formed by blackink, the structure of the embodiment calculates the resulting value RVC(RVM) and obtains the corrected data DCC (DCM) on the assumption thatdeep dots of cyan and magenta are in the on state (formed), prior to theprocessing of and after step S350. In case that dots are formed by blackink in a target pixel, the structure of the embodiment does not formdots of cyan or magenta ink therein, but sets the predetermined valuervck (rvmk) to the resulting value RVC (RVM), prior to calculation ofthe error (step S350).

After the calculation of the error, the process of error diffusion iscarried out (step S360). The error ERR obtained at step S350 isdistributed into peripheral pixels around the target pixel withpredetermined weights (see FIG. 12 in the first embodiment). After theerror diffusion process, the program goes to a next pixel and repeatsthe processing of steps S313 through S360 for the next pixel. Thesubsequent binary coding process for yellow ink (step S240) is differentfrom the binary coding process for cyan and magenta inks (steps S220 andS230) and is based on the systematic dither method. The binary codingprocess for yellow ink utilizes the same threshold matrix as that usedin the binary coding process for black ink. In case that dots are formedby black ink, no dots should be formed by yellow ink.

In the process of recording a multi-color image with a plurality of inksincluding black ink, when dots are formed by black ink, the structure ofthis embodiment considers dots to be also formed by cyan ink and magentaink and does not newly form dots of cyan and magenta in a target pixel.Furthermore no dots are formed by yellow ink in the target pixel. Thestructure of the embodiment prevents inks from being dischargedwastefully, thereby reducing the total amount of inks consumed. This isfavorable from the aspect of restricted amount of ink sprayed againstthe sheet of paper (ink duty). In case that dots are formed by blackink, the structure of the embodiment calculates the resulting values RVCand RVM for cyan ink and magenta ink on the assumption that dots arealso formed by cyan ink and magenta ink. In the on state of dots byblack ink, dots of cyan and magenta are not generally observed in thevicinity of the black dots. In an area where the respective color inksare sparsely recorded, for example, it is rather difficult to recorddots by cyan ink and magenta ink as well as dots by light cyan ink andlight magenta ink in the vicinity of dots by black ink. This favorablyimproves granularity of resulting images.

Although the above embodiment regards the relationship between black inkand cyan and magenta inks, the principle of the embodiment is notrestricted to cyan or magenta inks but is applicable to any chromaticcolor inks, such as yellow ink. The principle of the embodiment is alsoapplicable to another combination of inks discharged from the head,instead of the combination of CYM. The achromatic color ink may be alower-density ink, such as gray ink, other than black ink used in theabove embodiment. In case that the achromatic color ink having lowerdensity is used or in case that the blotting state of paper is varied,it is fair to change the resulting value RV when dots are formed by theachromatic color ink.

The following describes a fourth embodiment according to the presentinvention. Like the third embodiment, the structure of the fourthembodiment makes the results of determination of formation ornon-formation of dots by black ink reflect upon formation of dots bycyan ink and magenta ink. The only difference from the third embodimentis the technique of reflection. The flowcharts of FIGS. 25 through 27show the halftone processing carried out in the fourth embodiment.Although the description refers to the determination of formation ornon-formation with respect to only the dots by black ink and the dots bycyan ink, similar processing is carried out for magnet ink.

When the program enters the processing routine of FIG. 25, the positionof a target pixel is initialized (step S400). A concrete procedure setsthe value ‘0’ to both variables x and y, wherein x and y respectivelyrepresent the position in the horizontal direction and the position inthe vertical direction. The program then carries out the binary codingprocess for black ink based on a density K(x,y) of the achromatic colorin the target pixel and obtains a resulting value KRST (step S410). Thesystematic dither method is applied to the binary coding process ofblack ink as discussed previously. The flowchart of FIG. 26 shows theoutline of the processing according to the systematic dither method. Thedensity K(x,y) of the achromatic color is compared with a thresholdvalue Kdth read from a threshold matrix of discrete dither prepared inadvance (step S411). When K(x,y) is greater than the threshold value,the program determines formation of dots by black ink and turnsKdot(x,y) on (step S412). Otherwise the program turns Kdot(x,y) off(step S413). The resulting value is set equal to 255 in case offormation of dots by black ink and equal to 0 in case of non-formationof dots by black ink (steps S415 and S416).

After the binary coding process for black ink, the program obtainsmodified data Cx for cyan (step S420). The modified data Cx is obtainedby adding the tone data K(x,y) of black ink to tone data C(x,y) of thecyan component in the target pixel. Addition of the tone data of blackink to obtain the modified data Cx for the cyan component makes itdifficult to form dots of cyan ink in the place where dots are generallyformed by black ink (that is, in the place having a large K(x,y) value).A weighting coefficient KCW may be used to calculate the modified dataCx as expressed below:

C×=C(x,y)+K(x,y)·KCW

Although the weighting coefficient KCW is set equal to one in thisembodiment, the weighting coefficient KCW may be smaller than or greaterthan one. The weighting coefficient KCW smaller than one makes it easyto form dots of cyan ink on the average. The weighting coefficient KCWgreater than one, on the other hand, makes it difficult to form dots ofcyan ink on the average.

After correcting the tone data regarding the cyan component (step S420),the program carries out ternary coding for cyan ink (step S430). Theflowchart of FIG. 26 shows details of the ternary coding process forcyan ink. In brief, the on/off state of dots by the cyan ink C1 and thelight cyan ink C2 is determined, based on diffusion error-corrected dataCcr for the cyan component. This ternary coding process will bediscussed after the general procedure shown in the flowchart of FIG. 25.

After the ternary coding for the cyan component, an error occurring forthe cyan component is diffused into the peripheral pixels (step S450).The ternary coding process results in formation of dots by thehigher-density cyan ink C1, formation of dots by the lower-density lightcyan ink C2, or no formation of any dots. In any case, there generallyexists an error from the original tone data regarding the target pixel.The error is distributed into the peripheral pixels with some weightsspecified in FIG. 12 of the first embodiment.

The program then increments the variable x representing the position inthe primary scanning direction (moving direction of the head) by one(step S460), and subsequently determines whether or not the variable xrepresenting the position in the primary scanning direction exceeds anend Hmax in the primary scanning direction (step S470). In case that thevariable x does not exceed the end Hmax in the primary scanningdirection, the program returns to step S410 and repeats the processingof steps S410 through S470. In case that the variable x exceeds the endHmax in the primary scanning direction, on the contrary, the programinitializes the variable x to zero and increments the variable yrepresenting the position in the secondary scanning direction (feedingdirection of sheet of paper) by one (step S480). It is subsequentlydetermined whether or not the variable y representing the position inthe secondary scanning direction exceeds an end Vmax of the sheet ofpaper (step S490). When the variable y does not exceed the end Vmax, theprogram returns to step S410 and repeats the processing of steps S410through S490.

Referring to FIG. 27, the ternary coding process for the cyan componentis discussed. When the program enters the routine of FIG. 27, correcteddata Ccr for the cyan component is obtained by adding a diffused errorCdfer processed at step S450 to the modified data Cx obtained at stepS420 in the flowchart of FIG. 25 (step S431). As discussed above, themodified data Cx is obtained by adding the tone data of black ink to thetone data C(x,y) of the cyan component in the target pixel. Thecorrected data Ccr is given by adding the error diffused from theperipheral pixels to the modified data Cx and thereby represents thedensity of cyan ink to be realized in the target pixel. It is thendetermined whether or not dots are formed by black ink in the targetpixel. When Kdot(x,y) is off, that is, when no dots are formed by blackink, the corrected data Ccr is compared with a first threshold valueEdTh1 (step S433). In case that the corrected data Ccr is greater thanthe first threshold value EdTh1, the program determines formation ofdeep dot and turns the dots of the cyan ink C1 on in the target pixelCdot(x,y) (step S434), in order to realize the high density in thetarget pixel. In the on state of deep dots, a resulting value CRST forcyan ink is set equal to 255 (step S435). The program then specifies thedifference between the corrected data Ccr and the resulting value CRSTas a density error Cerr (step S440). The density error Cerr is aquantized error diffused into the peripheral pixels according to theerror diffusion process (step S450 in the flowchart of FIG. 25).

In case that the corrected data Ccr is determined to be not greater thanthe first threshold value EdTh1 at step S433, on the other hand, theprogram further compares the corrected data Ccr with a second thresholdvalue EdTh2, which is smaller than the first threshold value EdTh1 (stepS441). When the corrected data Ccr is not greater than the firstthreshold value EdTh1 but is greater than the second threshold valueEdTh2, the program determines formation of light dots, in order torealize the required density in the target pixel. The concrete proceduresets the light cyan ink C2 in the on state for the pixel Cdot (x,y)(step S442). In the on state of light dots, the resulting value CRST isset equal to 128 (step S443). In case that the corrected data Ccr isdetermined to be not greater than the second threshold value EdTh2, theprogram determines no formation of either deep dots or light dots andsets both the cyan ink C1 and the light cyan ink C2 in the off state forthe pixel Cdot(x,y) (step S444). In the off state of both deep dots andlight dots, the resulting value CRST is set equal to zero (step S445).

When no dots are formed by black ink in the target pixel (step S432),the program carries out the ternary coding process for the cyancomponent and forms either of the deep dots C1 and the light dots C2 ordoes not form any dots. When dots are formed by black ink in the targetpixel, on the other hand, the program proceeds to step S446. The programdetermines no formation of dots by the cyan ink C1 or the light cyan inkC2 (step S446), while setting the resulting value CRST equal to 255(step S447). Since dots have already been formed by black ink, theprocessing does not form any dots of the cyan component but sets theresulting value CRST on the assumption that deep dots of cyan areformed.

After determining formation or non-formation of deep dots and light dots(steps S434, S442, S444, and S446) and setting the resulting value CRST(steps S435, S443, S445, and S447), the program calculates the densityerror (step S440) as discussed previously.

In the fourth embodiment discussed above, formation or non-formation ofdots by black ink affects formation of deep dots and light dots by cyanink. Formation of dots by black ink makes it difficult to form dots bycyan ink in the vicinity of the black dots. Even when black ink and cyanink (or magenta ink) independently have high dispersibility, thestructure of the fourth embodiment effectively prevents dots of cyan inkfrom being formed adjacent to dots of black ink, thereby solving theproblem of obvious granularity. Since dots can be recorded by bothhigher-density ink and lower-density ink for cyan and magenta in thisembodiment, light dots are generally formed under such conditions if anydots should be formed. This attains the extremely high quality ofresulting images.

The following discusses the reason why the resulting value CRST for cyanink is set equal to 255 at step S447 in the above embodiment,irrespective of non-formation of dots by cyan ink. In this embodiment,the modified data Cx for cyan ink is obtained by adding the tone dataK(x,y) of black ink to the tone data C(x,y) of cyan ink at step S420 inthe flowchart of FIG. 25. The corrected data Ccr is accordinglycalculated at step S431 in the flowchart of FIG. 27 as: $\begin{matrix}{{Ccr} = {{Cx} + {Cdfer}}} \\{= {{C( {x,y} )} + {K( {x,y} )} + {Cdfer}}}\end{matrix}$

When dots are formed by black ink, the value ‘255’ is set to theresulting value CRST for cyan ink at step S447. The density error Cerrobtained by subtracting the resulting value CRST from the corrected dataCcr is accordingly expressed as: $\begin{matrix}{{Cerr} = {{Ccr} - {CRST}}} \\{= {{C( {x,y} )} + {K( {x,y} )} - {CRST} + {Cdfer}}}\end{matrix}$

In case that dots are formed by black ink, the processing sets theresulting value CRST for cyan ink equal to 255 without carrying out thedetermination of formation or non-formation of dots by cyan ink. In thiscase, the resulting value CRST reflects upon the resulting value in caseof formation of dots by black ink. Namely it is regarded as:

K(x,y)−CRST=Ker

When dots are formed by black ink, the error with respect to the blackink reflects upon formation of dots by cyan ink in the vicinity of theblack dots at step S450. When no dots are formed by black ink, theresulting value KRST for black ink is generally set equal to zero. Theprocess of adding the tone data K(x,y) of black ink (step S420 in theflowchart of FIG. 25) accordingly corresponds to the process of addingthe density error for the black ink. As in the case of formation of dotsby black ink, the on/off state of dots by black ink thus reflects uponthe on/off state of peripheral dots by cyan ink.

The following describes a fifth embodiment according to the presentinvention. Like the fourth embodiment, the structure of the fifthembodiment makes the on/off state of dots by black ink reflect uponformation of dots by deep cyan ink and light cyan ink. The hardwarestructure of the printing system and most part of the processing in thefifth embodiment are identical with those of the fourth embodiment. Theflowchart of FIG. 28 corresponds to the flowchart of FIG. 25 in thefourth embodiment. The fifth embodiment carries out the processing up tothe binary coding process for black ink (the processing of step S410)and the processing after the error diffusion process for cyan ink (theprocessing of step S450) in the fourth embodiment. Referring to FIG. 28,in the fifth embodiment, after the binary coding process for black ink(step S510), the program calculates modified data Cx. The calculation ofthe fifth embodiment is different from that of the fourth embodiment. Inthe fifth embodiment, the modified data Cx is obtained by adding theproduct of the difference between the tone data K(x,y) of black ink andthe resulting value KRST with respect to the dots of black ink and aweighting coefficient KCW to the tone data C(x,y) of the cyan componentin the target pixel. This is expressed as:

Cx=C(x,y)+{K(x,y)−KRST}·KCW

The program then carries out the ternary coding process for cyan ink(step S530). The flowchart of FIG. 29 shows the details of the ternarycoding process. The ternary coding process of the fifth embodiment isidentical with that of the fourth embodiment, except the procedure ofthe fifth embodiment does not include the process of determining whetheror not dots of black ink are in the off state (step S432 in theflowchart of FIG. 27) as well as the process of turning off the cyandots C1 and C2 (step S446) and the process of setting the resultingvalue CRST equal to 255 (step S447) in response to the negative answerof the decision point.

In this embodiment, the dots of cyan ink C1 and C2 are not always turnedoff in case that the dots of black ink are in the off state. Even whendots of black ink are formed, there exists a possibility of formation ofdots of cyan ink. The modified data Cx in the fifth embodiment is,however, obtained by adding the difference between the tone data K(x,y)of black ink and the resulting value KRST (more precisely, the productof the difference and the weighting coefficient KCW) to the tone dataC(x,y) of the cyan component. When dots of black ink are formed(KRST=255), the modified data Cx of cyan ink accordingly becomes smallerthan the tone data C(x,y) of cyan ink. This prevents dots of cyan inkfrom being readily formed.

The subtraction of the resulting value from the tone data of black inkin the calculation of the modified data Cx of the cyan component causesdots of cyan ink to be not generally formed in the vicinity of dots ofblack ink in case that dots of black ink have been formed (resultingvalue KRST=255). Since the difference between the tone data of black inkand its resulting value is added to the tone data of cyan ink in thefifth embodiment, the correction by the black ink is substantially equalto zero in a specific area, while having local effects to preventformation of dots by cyan ink in the vicinity of black dots. Theweighting coefficient KCW may be equal to one or alternatively smallerthan or greater than one. Like the fourth embodiment, the weightingcoefficient KCW smaller than one makes it easy to form dots of cyan inkon the average, whereas the weighting coefficient KCW greater than onemakes it difficult to form dots of cyan ink on the average.

As discussed previously, the effects of the dots by black ink are freelycontrolled by varying the weighting coefficient KCW. The weightingcoefficient close to one does not substantially affect the density ofcyan ink on the average, while having local effects. Although the fifthembodiment carries out the ternary coding process for cyan ink, when thehead can discharge at least three different inks of different densities,the fourth or higher coding of the tone expression may be realized. Thebinary or higher coding may also be realized in combination with themulti-tone expression by the overlapped dots of lower-density ink. Thestructure of the fifth embodiment is not restricted to cyan ink but isalso applicable to magenta ink and any other inks used in the printer.

In the fourth and the fifth embodiments discussed above, the on/offstate of dots by black ink affects the determination of whether or notdots are formed by cyan inks C1 and C2. The technique of the fifthembodiment may be applicable to the determination of formation of dotsby the cyan ink C1 and the light cyan ink C2 (or the magenta inks M1 andM2). The flowchart of FIG. 30 shows a halftone processing routine insuch a case. Steps of FIG. 30 other than steps S605 and S650 areidentical with those of the fourth embodiment. These identical stepshave like numerals in the lower two places and are not specificallydescribed here. Although FIG. 30 relates to the halftone processing forcyan ink, the same procedure is applicable to the higher-density ink andlower-density ink of other hues.

When the halftone processing routine of FIG. 30 starts, after theinitialization (step S600), the program determines the recordingdensities to be realized by the cyan ink C1 and the light cyan ink C2,that is, dot recording ratios C1(x,y) and C2(x,y), based on the tonedata C(x,y) of cyan ink (step S605). The dot recording ratios (recordingdensities) to be realized by these inks may be determined according tothe relationship of FIG. 13 discussed in the first embodiment. Theprogram then carries out the binary coding process for cyan ink C1 basedon the recording density C1(x,y) thus determined with respect to thecyan ink C1, and calculates a resulting value CRST (step S610). Theprocess of binary coding and calculating the resulting value for thecyan ink C1 may follow the procedure of the first embodiment oralternatively the procedure of the fourth embodiment.

Modified data Cx for cyan ink is then calculated from the resultingvalue CRST (step S620). The modified data Cx is obtained by adding theproduct of the difference between the recording density C1(x,y) of cyanink C1 and the resulting value CRST with respect to the dots of the cyanink C1 and a predetermined weighting coefficient WC to the recordingdensity C2(x,y) of the light cyan ink C2. This is expressed as:

Cx=C2(x,y)+{C1(x,y)−CRST}·WC

After calculating the modified data Cx, the program carries out thebinary coding process for the light cyan ink C2 (step S630). The binarycoding process may follow the procedure of the first embodiment oralternatively the procedure of the fifth embodiment. After the binarycoding process, the program carries out the error diffusion process forthe light cyan ink C2 (step S650) and distributes the error due to theon/off operation of the cyan ink C1 and the light cyan ink C2 to theperipheral pixels. The above processing is repeatedly executed for thewhole image (0<x≦Hnax, 0<y≦Vmax) (steps S660 through S690).

The sixth embodiment discussed above makes the on/off state of the dotsby either one of the higher-density ink C1 and the lower-density ink C2reflect upon the on/off state of the dots by the other, therebyrealizing the appropriate halftoning as a whole. Unlike the firstembodiment, the sixth embodiment does not necessarily turn off the dotsby the lower-density ink when the dots by the higher-density ink are on.Although formation of dots by the higher-density ink makes it ratherdifficult to form dots by the lower-density ink, the dots may be formedby the lower-density ink according to the requirements. This improvesthe accuracy of the halftone processing. For example, the technique ofthe sixth embodiment ensures the appropriate results when the density tobe realized by the deep dots and the light dots exceeds 100%. This isespecially advantageous when the density realized by the deep dots andthe light dots is varied, for example, with the quality of paper. In theprinting system where the two-time discharge of the ink of a fixeddensity at the same place creates the density difference from theone-time discharge, the number of times of discharging ink results information of at least two different dots having different densities perunit area. The technique of the sixth embodiment is applicable to thisstructure where an identical ink is used for both the cyan ink C1 andthe light cyan ink C2. The technique of the sixth embodiment is widelyapplicable to a variety of hardware structures as well as different inkdensities.

In the sixth embodiment, the calculation of the modified data Cx (stepS620) may be replaced by:

Cx=C2(x,y)+C1(x,y)−CRST·WC

The sum C2(x,y)+C1(x,y) corresponds to the density to be realized by thewhole cyan ink and may be regarded as the input tone data DS. Themodified data Cx can thus be obtained by:

Cx=DS(x,y)−CRST·WC

In this case, it is required to determine the recording density of onlythe higher-density ink, instead of determining the recording densitiesof both the higher-density ink and the lower-density ink at step S605.In this structure, however, the weighting coefficient WC only affectsthe resulting value CRST unless the weighting coefficient WC is equal toone.

In the above embodiments, the programs for controlling formation of dotsare stored in the printer driver 96 included in the computer 90. Theseprograms may, however, be stored in the printer 20. For example, in casethat the computer 90 sends image information written in a language, suchas PostScript, the printer 20 has a halftone module and other requiredelements. In the embodiments, the software realizing these functions isstored in a hard disk of the computer 90 and incorporated into theoperating system in the form of the printer driver at the time ofactivation of the computer 90. In accordance with another possibleapplication, the software may be stored in portable storage media(carriable storage media), such as floppy disks and CD-ROMs, andtransferred from the portable storage media to the main memory of thecomputer system or an external storage device. The software may betransferred from the computer 90 to the printer 20. Still anotherpossible application utilizes an apparatus for supplying the softwarevia a communication line. In this structure, the contents of thehalftone module may be transferred to either the computer 90 or theprinter 20 via the communication line.

The computer 90 may have an internal structure as shown in the blockdiagram of FIG. 31. The computer 90 includes a CPU 81 for executing avariety of arithmetic and logic operations according to programs inorder to control the actions related to image processing, and otherperipheral units mutually connected to one another via a bus 80. A ROM82 stores programs and data required for execution of the variety ofarithmetic and logic operations by the CPU 81. A RAM 83 is a memory,which various programs and data required for execution of the variety ofarithmetic and logic operations by the CPU 81 are temporarily read fromand written in. An input interface 84 receives input signals from ascanner 12 and a keyboard 14, whereas an output interface 85 sendsoutput data to the printer 20. A CRTC 86 controls signal outputs to aCRT 21 that can display color images. A disk controller (DDC) 87controls transmission of data from and to a hard disk 16, a flexibledisk drive 15, and a CD-ROM drive (not shown). The hard disk 16 stores avariety of programs that are loaded into the RAM 83 and executed, aswell as other programs that are supplied in the form of a device driver.A serial input-output interface (SIO) 88 is also connected to the bus80. The SIO 88 is connected to a public telephone network PNT via amodem 18. The (image processing apparatus 30 is connected with anexternal network via the SIO 88 and the modem 18, and can access aspecific server SV in order to download the programs required for imageprocessing into the hard disk 16. The computer 90 may alternativelyexecute the required programs loaded from a flexible disk FD or aCD-ROM.

The variety of programs executed in the above embodiments may berecorded on the recording media, such as flexible disks and CD-ROMs. Thecomputer 90 reads these programs by means of the disk drive 15, therebyrealizing the image recording method discussed above.

In the above embodiments, a predetermined voltage is applied to thepiezoelectric elements PE for a predetermined time period, in order todischarge both the low-density ink and the high-density ink. Anothermethod may, however, be applicable to discharge inks. The availableink-discharge techniques can be classified into two types; that is, themethod of separating ink particles from a continuous jet stream of inkand the on-demand method applied in the above embodiments. The formertype includes a charge modulation method that separates droplets of inkfrom a jet stream of ink by means of charge modulation and a micro-dotmethod that utilizes fine satellite particles produced in the process ofseparating large-diametral particles from a jet stream of ink. Thesemethods are applicable to the printing system of the present inventionthat utilizes inks of different densities.

The on-demand type, on the other hand, produces ink particles for therespective dot units according to the requirements. An available methodof the on-demand type, other than the method utilizing the piezoelectricelements applied in the above embodiments, arranges a heating body HT inthe vicinity of nozzles NZ of ink, produces bubbles BU by heating ink,and makes ink particles IQ discharged by the pressure of the bubbles BUas shown in FIGS. 32(A) through 32(E). Such on-demand type methods areapplicable to the printing system of the present invention that utilizesinks of different densities or plural types of dots having differentdiameters. The on-demand method is also applicable to the structure, inwhich dots of different densities are formed by discharging ink of aspecific density by a plurality of times.

INDUSTRIAL APPLICABILITY

The printing system, the image recording method, and the program productfor storing the image recording method according to the presentinvention discussed above enable multi-tone images to be printed on anobject, such as paper, with at least two different inks of differentdensities. The principle of the present invention is especially suitablefor recording high-quality images with a printing system, such as aprinter, with a less number of tones per dots.

What is claimed is:
 1. An image processing system which convertsoriginal image data to a distribution of at least two different dotshaving different densities per unit area on an object, said imageprocessing system comprising: recording density inputting means forspecifing a recording density to be realized by at least one of ahigher-density dot having a higher density per unit area and alower-density dot having lower-density per unit area, corresponding to atone signal of said original image data; a multi-one section comprising,a first multivaluing means for carrying out a multivaluing operationbased on the inputted recording density, and using the specifiedrecording density to determine whether one of the higher-density dothaving a higher density per unit area and the lower-density dot having alower density per unit area is to be formed, and a second multivaluingmeans for carrying out a multivaluing operation based on a result of thefirst multivaluing operation and the specified recording density todetermine whether the other one of said dots is to be formed.
 2. Animage processing system in accordance with claim 1, wherein saidrecording density inputting means specifies a recording density to berealized by one of the higher-density dot and the lower-density dot aswell as a recording density to be realized by the other one of thehigher-density dot and the lower density dot, corresponding to said tonesignal of the original image data, said system further comprising:recording density correcting means for obtaining correction data, whichreflects upon the recording density to be realized by the other one ofsaid dots, based on the result of the multivaluing operation withrespect to the selected one of said dots, in order to correct therecording density to be realized by the other one of said dots, whereinsaid second multivaluing means activates said multivaluing operation forforming the other one of said dots, under the cause that said reflectedrecording density to be realized by the other one of the dots is saidcorrected recording density.
 3. An image processing system in accordancewith claim 2, wherein said recording density correcting means carriesout the correction to reflect a local recording density based on theresult of the multivaluing operation by said first multivaluing meansand to cause a mean recording density in a specified range to besubstantially equal to the recording density to be realized by theselected one of the higher-density dot and the lower-density dot.
 4. Animage processing system in accordance with claim 3, wherein saidrecording density correcting means adds a difference between therecording density to be realized by the selected one of the dots and therecording density realized by the result of the multivaluing operationby said first and said second multivaluing means to the recordingdensity to be realized by the other one of the dots.
 5. An imageprocessing system in accordance with claim 1, wherein said firstmultivaluing means activates said multivaluing operation for forming theselected dot, prior to the multivaluing operation for the other one ofsaid dots, and said second multivaluing means activates saidmultivaluing operation, in case the selected dot is determined to beformed as a result of the multivaluing operation by said firstmultivaluing, said system further comprising: error diffusion means forcomputing a difference between a first density corresponding to the tonesignal and a second density based on the multivaluing operation of saidfirst and second multivaluing means as a density error, and distributingthe density error to peripheral pixels in the vicinity of a currenttarget pixel of the image data, in order to reflect upon themultivaluing operation by said first and second multivaluing means withrespect to the peripheral pixels.
 6. An image processing system inaccordance with claim 5, wherein said at least two different dots havingdifferent densities per unit area are formed with a chromatic color ink,said system further comprising: third multivaluing means for carryingout a multivaluing operation for an achromatic color ink and using saidrecording density to form the dot of the achromatic ink based on therecording density of an achromatic color ink to form said image with achromatic color ink; and modification means for activating said firstand said second multivaluing means, and said error diffusion means,based on the result of multivaluing operation for said achromatic colorink by said third multivaluing means.
 7. An image processing system inaccordance with claim 1, wherein said first multivaluing means carriesout the multivaluing operation for forming the dot having a higherdensity per unit area.
 8. An image processing system in accordance withclaim 1, wherein said first multivaluing means carries out themultivaluing operation for forming the dot having a lower density perunit area.
 9. An image processing system in accordance with claim 1,wherein said first multivaluing means activates said multivaluingoperation through a dither method.
 10. An image processing system inaccordance with claim 9, wherein said first multivaluing means uses athreshold matrix of discrete dither.
 11. An image processing system inaccordance with claim 1, wherein said at least two different dots havingdifferent densities per unit area are formed in different diameters. 12.An image processing system which converts original image data to adistribution of at least two different dots having different densitiesper unit area on an object, said image processing system comprising:input means for successively receiving a tone signal of each targetpixel included in said original image data; recording density inputtingmeans for specifying a first dot recording density defined as a tonevalue to be realized by a first dot selected among said at least twodifferent dots having different densities per unit area, correspondingto the tone signal; first multivaluing means for determining whether ornot the first dot is to be formed, based on the first dot recordingdensity; correction signal computing means for computing a correctionsignal by adding quantization errors distributed from peripheralprocessed pixels in the vicinity of said target pixel to the input tonesignal; second multivaluing means for, when said first multivaluingmeans determines no formation of the first dot, determining whether ornot a second dot having a different density per unit area from that ofthe first dot is to be formed, based on the correction signal; and errordiffusion means for computing a quantization error, which is adifference between the correction signal and a tone value realized bythe formed dots, as a density error, based on the results of themultivaluing operation by said first multivaluing means and said secondmultivaluing means, and distributing and diffusing the computed densityerror to peripheral pixels in the vicinity of said target pixel.
 13. Animage processing system in accordance with claim 12, wherein said firstmultivaluing means carries out the multivaluing operation for formingthe dot having a higher density per unit area.
 14. An image processingsystem in accordance with claim 12, wherein said first multivaluingmeans carries out the multivaluing operation for forming the dot havinga lower density per unit area.
 15. An image processing system inaccordance with claim 12, said system further comprising: errordiffusion means for computing a difference between the recording densityspecified by said recording density inputting means and a recordingdensity realized by said at least two different dots having differentdensities per unit area as a density error, based on the multivaluingoperation of dot formation by said first multivaluing means and saidsecond multivaluing means, and distributing the density error toperipheral pixels in the vicinity of a current target pixel of dotformation, in order to reflect upon the multivaluing operation of dotformation with respect to the peripheral pixels by said secondmultivaluing means.
 16. An image processing system in accordance withclaim 12, wherein said at least two different dots having differentdensities per unit area are formed with a chromatic color ink, saidsystem further comprising: third multivaluing means for carrying out amultivaluing operation for an achromatic color ink, so as to determinewhether or not the achromatic dot is to be formed by said achromaticcolor ink; and determination modifying means for activating said firstmultivaluing means, said second multivaluing means, and said errordiffusion means, based on the result of multivaluing operation for saidachromatic color ink by said third multivaluing means.
 17. An imageprocessing system in accordance with claim 16, wherein when said thirdmultivaluing means determines formation of the achromatic dot, thecalculation of the density error by said error diffusion means iscarried out by a technique that is different from the technique appliedfor the calculation when said first multivaluing means determinesformation of the selected dot.
 18. An image processing system inaccordance with claim 16, wherein said chromatic color ink is at leastone of cyan and magenta.
 19. An image processing system in accordancewith claim 16, wherein said chromatic color inks are cyan and magentainks, said achromatic color ink is black ink, said third multivaluingmeans determining whether or not the black dot is to be formed by saidblack ink, when said third multivaluing means determines formation ofthe black dot by said black ink, said determination modifying meansassuming that said first multivaluing means determines formation of aselected dot among said at least two different dots having differentdensities per unit area by said cyan and magenta inks and activatingsaid second multivaluing means and said error diffusion means.
 20. Animage processing system in accordance with claim 12, wherein said secondmultivaluing means comprises: local effect computing means forcalculating a local effect from the recording density of the selecteddot, which is subjected to determination of multivaluing operation bysaid first multivaluing means, and a printing density realized by theselected dot; and recording density correcting means for correcting therecording density to be realized by the other dot by taking into accountthe local effect, so as to affect the determination of dot formationwith respect to the other dot.
 21. An image processing system inaccordance with claim 20, wherein said local effect computing meanscalculates a difference between the recording density of the selecteddot and the printing density realized by the selected dot as a localerror, said recording density correcting means adding a product of thelocal error and a predetermined weight to the recording density to berealized by the other dot, so as to affect the determination of dotformation with respect to the other dot.
 22. An image processing systemwhich converts original image data to a distribution of at least twodifferent dots having different densities per unit area by a chromaticcolor ink as well as an achromatic dot by an achromatic color ink, saidimage processing system comprising: density specifying means forspecifying a density to be realized by said chromatic color ink and adensity to be realized by said achromatic color ink, corresponding to atone signal of said image data; achromatic multivaluing means forcarrying out a multivaluing operation for said achromatic color ink,based on the specified density to be realized by said achromatic colorink, and using said specified density to form said achromatic dot;density correcting means for obtaining correction data, which reflectsupon the density to be realized by said chromatic color ink, based onthe result of the multivaluing operation with respect to said achromaticcolor ink, in order to correct the density to be realized by saidchromatic color ink; and chromatic multivaluing means for carrying out amultivaluing operation with respect to said at least two different dotshaving different densities per unit area, based on the corrected densityto be realized by said chromatic color ink, and using the correcteddensity to form said at least two different dots.
 23. An imageprocessing system in accordance with claim 22, said system furthercomprising: error diffusion means for computing a difference between arecording density corresponding to the tone signal and a recordingdensity realized by the dots of said achromatic color ink and saidchromatic color ink as a density error, based on the result ofmultivaluing operation by said achromatic multivaluing means and saidchromatic multivaluing means, and distributing the density error toperipheral pixels in the vicinity of a current target pixel of saidimage data, in order to reflect upon the multivaluing operation withrespect to the peripheral pixels by said achromatic multivaluing meansand said chromatic multivaluing means.
 24. An image processing system inaccordance with claim 16, wherein said density correcting means carriesout the correction to reflect a local achromatic color ink density basedon the result of the multivaluing operation with respect to the dot ofsaid achromatic color ink and to cause a mean achromatic color densityin a specified range to be substantially equal to a density to berealized by said achromatic color ink.
 25. An image processing system inaccordance with claim 24, wherein said density correcting means adds adifference between the density to be realized by said achromatic colorink and the density of said achromatic color ink realized by the resultof the multivaluing operation by said achromatic multivaluing means tothe density to be realized by said chromatic color ink.
 26. An imageprocessing method for converting original image data to a distributionof at least two different dots having different densities per unit areaon an object, said image processing method comprising: inputting arecording density to be realized by at least a selected one of ahigher-density dot having a higher density per unit area and alower-density dot having a lower density per unit area, which are bothincluded in said at least two different dots having different densities,corresponding to a tone signal of said original image data; carrying outa multivaluing operation based on the inputted recording density, andusing the inputted recording density to form the selected one of saiddots; making a result of the multivaluing operation reflect upon arecording density to be realized by the other one of said dots; causingthe other one of said dots to be subjected to a multivaluing operationaccording to the reflected recording density; and using the reflectedrecording density to form the other one of said dots.
 27. A computerprogram product comprising a computer storage medium having a computerprogram embedded in the computer storage medium for implementing animage processing method for converting original image data to adistribution of at least two different dots having different densitiesper unit area on an object, the computer program performing the stepsof: inputting a recording density to be realized by at least a selectedone of a higher-density dot having a higher density per unit area and alower-density dot having a lower density per unit area, which are bothincluded in said at least two different dots having different densities,corresponding to a tone signal of said original image data; carrying outa multivaluing operation based on the inputted recording density, andusing the inputted recording density to form the selected one of saiddots; making a result of the multivaluing operation reflect upon arecording density to be realized by the other one of said dots; causingthe other one of said dots to be subjected to a multivaluing operationaccording to the reflected recording density; and using the reflectedrecording density to form of the other one of said dots.
 28. An imageprocessing system which converts original image data to a distributionof at least two different dots having different densities per unit areaon an object, said image processing system comprising: a recordingdensity inputting unit for specifing a recording density to be realizedby at least one of a higher-density dot having a higher density per unitarea and a lower-density dot having a lower density per unit area,corresponding to a tone signal of said original image data; a multi-tonesection comprising, a first multivaluing unit for carrying out amultivaluing operation based on the inputted recording density, andusing the specified recording density to determine whether one of thehigher-density dot having a higher density per unit area and thelower-density dot having a lower density per unit area is to be formed,and a second multivaluing unit for carrying out a multivaluing operationbased on a result of the first multivaluing operation and the specifiedrecording density to determine whether the other one of said dots is tobe formed.